Developing rubber compounds containing high sulfur levels has been a serious problem for the rubber industry, and one which has received a great deal of attention. Problems attributable to high levels of sulfur in the rubber include migration of the sulfur to the surface of the rubber stock, commonly referred to as “bloom”, which causes decrease of tack at the surface of the rubber stock.
Compounds containing high sulfur levels can exhibit problems with sulfur bloom on the surface of the unvulcanized rubber. This surface layer of sulfur crystallizes, causing a loss of building tack which can cause problems in tire building, such as decreased calendar speed and problems in storage of components.
Numerous modifications of standard rubber processing techniques have been utilized to minimize the sulfur bloom tendencies. These prior methodologies include: the use of insoluble sulfur in the compound; limiting the compound mixing temperatures during the sulfur addition stage; and minimizing the heat history that the compound is exposed to during processing. However, these modifications have led to mixed results.
For example, insoluble sulfur is formed by rapidly quenching molten sulfur that is above 159° C. (preferably 200 to 250° C.), and consists primarily of long chain sulfur molecules and a lesser amount of soluble S8 rings. But there is a tendency for the long chain molecules to revert to the more stable soluble form if exposed to higher temperatures, long storage times, and/or hostile storage environments. To reduce this tendency, commercial insoluble sulfur products contain a stabilizer.
When insoluble sulfur is mixed in a rubber compound, it exists as more or less discreet particles of varying size in the rubber phase. However, above about 118° C., substantial reversion to the soluble sulfur form occurs, thereby resulting in sulfur bloom.
Another approach to the sulfur bloom problem has been to use a low-sulfur rubber compound. Low-sulfur rubber compounds may be obtained by use of suitable sulfur vulcanizing agents, for example, elemental sulfur (free sulfur), optionally in combination with a sulfur donating vulcanizing agent, for example, an amine disulfide, polymeric polysulfide, or sulfur olefin adducts. However, with this approach, adhesion of the rubber compound to reinforcement such as steel wire, steel cord, or the like may be compromised.
Accordingly, there is a need for a rubber composition in which sulfur bloom, and the resulting loss of surface tack, is reduced or avoided over a wider temperature range, without compromising adhesion of the rubber compound to a reinforcement material.